A helical wrapping technique that has now been practically abandoned, used to provide for a film to be stratched directly on a palattized load that was itself generally in the form of a rectangular parallelepiped. The film was paid out from a vertical axis reel fitted with a brake. An end of the film was initially fixed to the load and the load was then caused to rotate about a vertical axis. The film was stretched to a desired value by appropriately braking the real.
Unfortunately, since the load was not cylindrical, but generally in the form of a rectangular parallelepiped, the radial distance to be covered by the film varied with angle, so that rotating the load at constant angular speed and applying constant braking force did not ensure that the film was elongated uniformly. In addition, stretchable film is subject to relaxation such that over a period of substantially 48 hours, the resilient return force relaxes by substantially 50%. Thus, with that type of machine, the stretch forces were limited by those that could be withstood by the load, while the elastic return forces that remained to ensure cohesion of the load during handling and transport were substantially smaller. It therefore became necessary to abandon that type of machine in favor of winding machines of a type that includes a prestretching device having a plurality of motor-driven rollers rotating at different peripheral speeds. At the outlet from the rollers of the prestretching device, the film was wound onto the load to be wrapped with little or no mechanical tension, as explained below with reference to FIG. 1.
In patent application No. 92 10254 (published under the number FR 2 695 102), the Applicant describes a method of wrapping a load with a previously-stretched film, the film being put into place only after the film has been allowed time to relax under mechanical tension. As explained below with reference to FIG. 2, that patent application mentions that it is thus possible to obtain increased residual elongation of the film after relaxation, thereby reducing consumption thereof.
As explained below with reference to said FIG. 2, the Applicant has since discovered that subsequent traction applied to a prestretched film which has relaxed under the prestretching mechanical tension, gives rise to elongation that is less than that generated under the same conditions using an identical film whose stretching has been followed by relaxation under a mechanical tension that is significantly less than the prestretching tension.